The present invention relates to a CCD control system for prevention of CCD breakage, and an endoscope which includes the CCD control system.
CCD (Charge Coupled Device) is a device that can transform an optical image of a subject formed on a receiving surface into electric signals with an opt-electric conversion. Usually, the electric signals are processed by a signal processing section incorporated in the CCD control unit outside the device, and are output as image signals. It is necessary to supply various kinds of power voltages and clock pulses to drive the CCD. One of the power voltages is a substrate bias voltage (hereinafter referred to as Vsub). Vsub is generated by a Vsub generation circuit. CCDs fall into two classes: an internal Vsub generation type that CCD includes a Vsub generation circuit, and an external Vsub generation type that CCD is supplied Vsub by a Vsub generation circuit incorporated in the CCD control unit outside the CCD.
An electronic endoscope is configured with an insertion tube which is inserted into a human cavity, an operation unit, a scope unit which includes cables and connectors for connection with a processor unit, and the processor unit which has an electric power supply and a light source supplied to the scope unit, and functions such as processing image signals from the scope unit to output to a monitor. Intensive research and development have been made to make the insertion tube as thinner as possible to reduce pain and suffering of patients and to enable observation inside of canals of a human body. Therefore, it is desirable that the smaller CCD is placed at the distal end of the insertion tube of the scope unit.
Because an external Vsub generation type CCD does not require a space for placing a Vsub generation circuit inside the device, it is possible to make the device smaller than an internal Vsub generation type CCD. On the other hand, the external Vsub generation type CCD requires a Vsub supply line from the CCD control unit which has the Vsub generation circuit extended to the CCD. Usually the CCD control unit is located in the scope, but near the processor unit, namely, near the connectors. So, the Vsub supply line is extended to the CCD installed at the distal end of the insertion tube through the scope as well as the other signal lines.
Therefore, in the case of an electronic endoscope with the external Vsub generation type CCD, it is highly possible that Vsub is not input to the CCD or the voltage level lowers, and the other signal, such as a clock pulse, is input, compared with in the case of an electronic endoscope with the internal Vsub generation type CCD. As a result, breakage of the external Vsub generation type CCD more likely caused by a latchup, etc. than the internal Vsub generation type CCD. Possible reasons why Vsub is not input may be a Vsub output error of the Vsub generation circuit such as output power loss of the circuit, a Vsub supply error from the Vsub generation circuit to the CCD such as short-circuit, wire breakage, bad electrical contact in a Vsup supply line. In the case of the electronic endoscope with the external Vsub generation type CCD, it is desirable that the other signal inputs than Vsub are stopped instantaneously when a Vsub output error is detected so as to prevent a latchup, etc.
Usually, power voltage around 15 V is supplied to the the CCD. It is also possible that the CCD breakage occurs when an error happens and the power voltage is not supplied. Namely, when minus power, for example minus power of a vertical clock pulse, is supplied while power voltage is not supplied to the CCD, a latchup is caused.
On the other hand, in order to detect a Vsub supply error and a power voltage supply error, it is possibly required to place two devices to monitor the Vsub and power voltage respectively near the CCD, to lay signal lines to transmit monitor signals to the CCD control circuit, and further to place circuits to receive the monitor signals in the CCD control circuit. But in that case, the number of devices near the CCD and the number of signal lines increase. As a result, the diameter of the insertion tube of the scope unit becomes larger, and the CCD control circuit becomes complex because receiving circuits of the monitor signals are also required.